Apothecary's use painkillers and various drugs and techniques to get Marines on their feet. They enhance and aid the superhuman biology of the Marine to get them back in the fight. Painboyz don't use painkillers, except maybe a blow to the head to knock the patient out, or other medicinal drugs. They crudely fix the Ork to the point where their natural healing takes over, plus they also like to enhance the Ork by adding biological or mechanical enhancements. In-game I believe the Apothecary should be a Tactical equivalent with healing and wargear limitations. They should have the Heal-Over-Time effect and possibly an Area-Of-Effect/Squad ability. Now the Painboy, I think, should heal up to a point then the Ork's natural health regen takes over. They should also have the ability to temporarily enhance an Ork, though it should be uncommonly used, during healing. Maybe even have a very limited ability to enhance an Ork until they die, possibly from the enhancement!